This invention relates to apparatus and methods for processing samples, such as wafers or substrates, with a process, such as a process directed towards chemical vapor deposition (CVD) or plasma enhanced chemical vapor deposition (PECVD). In particular, the invention provides apparatus and methods for improving the handling, cleanliness, and exposure of those samples within CVD and PECVD processes.
As used herein, a "sample" generically describes a substrate, such as a glass panel or a silicon wafer, which is suitable for deposition techniques, for example PECVD. The sample has a front side, which is treated to receive one or more coating depositions onto the surface, and has a back side, which is generally used to support and manipulate the sample.
Systems which deposit semiconductor films onto samples are well-known today and are utilized in a range of scientific fields. The semiconductor industry, for example, has developed a number of complex PECVD systems to manufacture devices such as thin film transistors (TFTs), liquid crystal displays (LCDs), flat panel displays (FPDs), solar cells, photodetectors, and other integrated circuits.
Typically, these systems include one or more evacuated process chambers that are used to expose the sample to reactive plasmas. The chambers include an electrode pair, to create radio-frequency (RF) discharges between the electrodes, and gas outlets, to pass selective gases, such as silane, between the electrodes. Once the gas passes within the RF electric field, it forms a reactive plasma which deposits onto surfaces in contact with the plasma, including the front side of the sample. By controlling the power and the frequency of the RF energy, the electrode spacing, and the exposure-time during which the sample contacts the reactive plasma, a selective thin film is deposited on the sample surface.
Generally, one process chamber is used for one particular coating or thin film deposition. If a sample is to have more than one type of film deposited on its surface, the system typically has more than one process chamber, and has a robot transport mechanism to move the sample between the several evacuated process chambers. Such a system also has an evacuated intermediate chamber, which serves to access the several process chambers and to reduce the amount of unwanted spurious gas or other contaminants which travel between process chambers. For example, once a film is deposited on the sample in a first chamber, it is transported by the robot through the intermediate chamber and to the second process chamber. The second process chamber is then sealed from the intermediate chamber and the additional film is deposited on the sample.
In certain prior systems, the intermediate chamber also conveniently provides an access port to the system. That is, the samples are loaded into the system and individually processed through the intermediate chamber before deposition in the respective process chambers; and finished samples are removed through the intermediate chamber.
One recurring problem in these systems is contamination. During a deposition cycle, non-uniformities in the RF electric field can result in the formation of floating powder-like substances in the plasma which thereafter fall onto the sample, creating defects in the sample's film structure and consequently in its semiconductor characteristics. Furthermore, plasma-generated films tend to form and accumulate on every chamber surface in contact with the reactive plasma. After the process chamber cycles through several deposition operations, the accumulated films often flake apart, producing minute particles that again fall on and contaminate the sample surface.
This is a severe problem; and the known solutions are not cost effective. One practice is simply to reduce the effective manufacturing yield. For example, portions, or even all, of the sample are discarded if it is contaminated or otherwise degraded by the powder or particles. Another known practice is to mechanically clean the process chamber often, so that the unwanted films on the chamber wall do not accumulate to a thickness that can flake off onto the sample. The chamber is typically flushed with a cleansing gas and then opened to clean and remove the accumulated particles. Yet another practice utilizes etching gases. These gases are injected through the process chambers between depositions, to etch the accumulated films on the chamber walls, thereby lengthening the time interval between the more thorough mechanical cleanings.
These practices, however, are wasteful. They reduce production yield, which costs the manufacturer valuable resources in an ever-competitive market. In the manufacture of Flat Panel Displays (FPDs), for example, forty percent of the sales costs are in materials. Thus production yields are important. The loss of even one sample due to contamination reduces revenue significantly.
Cleaning process chambers mechanically is also costly. The system is shut-down during the cleaning procedures, since it cannot effectively continue to deposit films onto samples at the same time; and there is an additional delay to re-evacuate the chamber and to re-qualify the process. Likewise, auto-cleaning techniques, which utilize etching gases, inhibit production yield rates by reducing the throughput of the system. These etching techniques effectively double the time between sample depositions, since etch rates and deposition rates are approximately equal. Thus the speed and effectivity of the production line are handicapped by contamination problems.
It is, accordingly, an object of the invention to provide improvements to CVD and PECVD systems and methods.
Another object of the invention is to provide improvements to systems which expose samples to reactive plasmas, and in particular to improve the production yield for uncontaminated samples.
Yet another object of the invention is to provide PECVD apparatus and methods which reduce the contamination on samples used in semiconductor devices.
Still another object of the invention is to provide improved apparatus and methods for exposing samples to reactive plasma which are relatively low in cost, and reliable in operation.
More particularly, it is an object of the invention to provide improved apparatus and methods for handling samples within a PECVD system.
These and other objects of the invention will be apparent in the description which follows.